WEEKEND SCIENCE: Experimenting with afterimages 後像實驗

This April 25, 2007 photo shows a human eye. 某人的一隻眼睛，攝於二ＯＯ七年四月二十五日。照片：法新社

PHOTO: AFP

If you have ever stared at a light bulb for more than a few seconds, you may have noticed that you could still see an outline of the image for a few seconds after you stopped looking it. The outline you saw is known as an afterimage, or ghost image. In today’s experiment we will take a closer look at afterimages and learn about the features of the eye that cause them.

What you will need: a flashlight, some cardboard and some adhesive tape.(JOHN PHILLIPS, STAFF WRITER)

Step 1: Use the cardboard to block out most of the light from the flashlight. The remainder of the light should make a recognizable shape, like a square, triangle or star.

Step 2: Stare at the flashlight for about thirty seconds. Make sure you use a flashlight with low luminosity, otherwise you could damage your eyes.

Step 3: Look at a wall. You should notice that the afterimage persists for a few seconds.

Step 4: Hold up your hand and look at it. You will see the afterimage on your hand and it will appear larger than when it was on the wall.

Step 5: Cover your left eye and stare at the flashlight with your right eye. Uncover your left eye and look at the wall and you will see the afterimage. However, when you cover your right eye you won’t see an afterimage.

方法

步驟一：用厚紙板擋住手電筒大部分的光源，讓剩下的部份變成可辨認的形狀，例如：正方形、三角形或星形。

步驟二：注視手電筒約三十秒。務必選用低光度的手電筒，否則你的眼睛可能會受傷。

步驟三：盯著一面牆看，你會看見後像在牆上存留數秒。

步驟四：把手舉起來，然後注視著它。你會在手上看到後像，而且它會比在牆上時來得大。

步驟五：遮住左眼，只用右眼注視手電筒的光線。鬆開手用兩眼一起注視牆壁，你會看到後像；可是當你遮住右眼時，你就看不到後像了。

HOW IT WORKS

Your retina works in the same way as photographic paper. When it is exposed to light, the light creates an image on it. This image is then relayed to the brain. When you stare at a bright light for a long time, the photosensitive cells in your retina desensitize to the brightness and stop working as efficiently. When you look at the wall, the desensitized cells create a temporary silhouette. After a few seconds, they readjust and you see the wall normally.

VOCABULARY 今日單字

1. afterimage n.

後像 (hou4 xiang4)

2. recognizable adj.

可辨認的 (ke3 bian4 ren4 de5)

3. persist v.i./v.t.

存留(cun2 liu2)

4. photographic adj.

攝影的 (she4 ying3 de5)

5. retina n.

視網膜 (shi4 wang3 mo2)

6. silhouette n.

輪廓 (lun2 kuo4)，剪影 (jian2 ying3)

In step 5, the reason you could only see the afterimage in the uncovered eye is because it occurs at the point of input (the retina) and not the point of process (the brain).